Today's wet stations and spray processors require several steps to remove photoresist from a wafer. The standard wet bench configuration includes a couple of isopropyl alcohol (IPA) rinse steps, but the final rinse step, often called the IPA dry step, is a critical step. In batch applications, the final rinse is provided by a thin film of clean, distilled IPA, which condenses on the wafer surface from the vapors located above the IPA liquid. In single wafer applications, the IPA can be directly sprayed onto the wafer as a liquid. This thin layer of IPA solvent prepares the wafer for final drying.
As geometries on the wafer get increasingly smaller, the IPA dry step becomes even more critical. Critical designs are pushing towards IPA rinse only to reduce damage to wafers with extremely small pitch (line spacing), especially on gate-level applications where metal contamination is extremely critical and cannot be tolerated. Therefore, the purity of IPA plays a larger role, and making sure there are no impurities transferred to the wafer from the IPA becomes extremely important.
A low capacity version of an IPA purifier using a standard, surface-modified microporous membrane cleaned with IPA is known for low temperature and high temperature applications, but the ion-exchange capacity is very low. If severe metal spikes occurred, this filter would have difficulty with adequate capacity.
Semiconductor manufacturers purchase very clean IPA, but then transfer it through the fab and tool via stainless steel lines. This can cause the IPA to pick up iron, nickel and other trace metals from the stainless steel which, unless removed, will be deposited on the wafer during the spray application. Because these metals are present at trace levels, it is difficult to measure the amount of contamination in the fluid stream; instead, the amount of contaminant is determined as metal contamination on the wafer, typically by TXRF scans, when it has already negatively impacted gate and yield performance.
Some microporous filters have been modified with ion-exchange material and used to remove metal contaminants from deionized (DI) water. These filters show high removal of metals without selectivity to any one metal. This technology is limited because some sulfonic acid functionality can break down and be shed over time due to degradation of the ion-exchange (IEC) media. This breakdown, which has also been observed in pellet media, causes non-volatile residue (NVR) and sulfonic acid ions to shed downstream of the media. The shed residue can then be deposited on a wafer, which can be a problem for semiconductor manufacturers or end-users.
U.S. Pat. No. 7,172,694, issued Feb. 6, 2007 to Bortnik, discloses a filter assembly adapted for use in filtering fluid flow in turbomachinery. The filter assembly includes a cylindrical housing, and a filter element disposed within the housing. The housing is adapted for fluid connection to a turbomachine. The filter element is adapted to filter fluid passing to the turbomachine. The filter element includes a fluid permeable core element defining a central core element flow channel through the filter element, a fluid permeable ion exchange resin layer disposed about the core element and adapted to remove mineral and organic acids from the fluid passing through the filter element, and a pleated filter media disposed about the ion exchange resin layer and core element. In another embodiment, the filter element has the pleated filter media disposed about the core element, and the fluid permeable ion exchange resin layer disposed about the core element and pleated filter media.
There is a continuing need for a point-of-use alcohol purifier that provides higher purity solvent for a wafer spray process tool and thereby provides better semiconductor device performance and process yield.